Semiconductor devices, such as discrete power MOSFETs typically use thick metal films (eg. 8μ of Aluminum or other suitable metal) in order to carry the large currents which may be associated with these devices. In order to etch such thick films it is normal to use wet etch, as dry etching has many problems. Thus:
a) Dry etching requires an extremely thick resist. That is, aluminum to resist selectivity is normally low except for the latest generation, and extremely expensive etchers. Further, a very thick resist has an unrealistically long exposure time on steppers and has poor resolution.
b) The use of dry etching is very costly due to the very low throughput as dry metal etch rates are not very high.
c) Any silicon nodules in an aluminum layer will etch more slowly than the aluminum. With thick aluminum films, this can cause pillars of unetched Al remaining after the etch process.
A serious disadvantage of wet etching is that, unlike dry etching, it is isotropic (ie. it etches laterally as well as vertically). (See FIGS. 1 to 3.) This means that the Aluminum has to be formed with a very large overlap of the contacts, to ensure that the Aluminum is not etched away from over the contacts which would be a reliability concern. Due to the large overetch normally used, an overlap of say, 16 μm is common with an 8 μm thick film for aluminum on a silicon wafer. This large overlap wastes a lot of silicon area. As silicon performance is increasing and die are shrinking, this wasted silicon area is becoming a larger percentage of the ultimate die singulated from the wafer.